I. Field of the Invention
The present invention relates to digital communications. More particularly, the present invention relates to a novel and improved system and method for monitoring the load in a CDMA system.
II. Description of the Related Art
In the field of code division multiple access (CDMA) wireless communication, a wideband frequency channel is shared by multiple communication devices, with each communication device employing a different pseudorandom noise (PN) spreading code. In a typical CDMA wireless communication system, a first frequency band is used for forward channel communications (from the base station to the mobile station), while a second frequency band, different from the first frequency band, is used for reverse channel communications (from the mobile station to the base station). An example of such a system is given in U.S. Pat. No. 4,901,307 entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters,xe2x80x9d issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference.
Fundamental to the concept of maximizing system capacity in a CDMA wireless communication system as described above is the process of power control. The output power of subscriber units must be controlled to guarantee that enough signal strength is received at the base station and to maintain good quality audio while minimizing the potential for interference. Since a CDMA wideband channel is reused in every cell, self interference caused by other users of the same cell and interference caused by users in other cells is the most limiting factor to the capacity of the system. Due to fading and other channel impairments, maximum capacity is achieved when the signal-to-noise ratio (SNR) for every user is, on the average, at the minimum point needed to support xe2x80x9cacceptablexe2x80x9d channel performance. Since noise spectral density is generated almost entirely by the interference from other users, all signals must arrive at the CDMA receiver with the same average power. In the mobile propagation environment, this is achieved by providing dynamic power control of the mobile station transceiver. Power control guards against changes in system loading, jamming, slow and fast variations in channel conditions, and sudden improvements or degradations in the channel (shadowing).
Power control of the mobile station transmitter consists of two elements: open loop estimation of transmit power by the mobile station, and closed loop correction of the errors in this estimate by the base station. In open loop power control, each mobile station estimates the total received power on the assigned CDMA frequency channel. Based on this measurement and a correction supplied by the base station, the mobile station transmitted power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. All mobile stations use the same process and arrive with equal average power at the base station. However, uncontrolled differences in the forward and reverse channels, such as opposite fading that may occur due to the frequency difference and mismatches in the receive and transmit chains of the mobile station, can not be estimated by the mobile.
To reduce these residual errors, each mobile station corrects its transmit power with dosed loop power control information supplied by the base station via low rate data inserted into each forward traffic channel. The base station derives the correction information by monitoring the reverse CDMA Channel quality of each mobile station, compares this measurement to a threshold, and requests either an increase or decrease depending on the result. In this manner, the base station maintains each reverse channel, and thus all reverse channels, at the minimum received power needed to provide acceptable performance. An example of a communication system employing the open loop and closed loop power control methods described above is given in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMethod And Apparatus For Controlling Transmission Power In A CDMA Cellular Mobile Telephone System,xe2x80x9d assigned to the assignee of the present invention, and incorporated herein by reference.
In a CDMA wireless communication system as described above, a predetermined number of radio frequency resources, such as transceivers and channel modulator/demodulators (modems) are located at each base station. The amount of resources allocated to a base station depends upon the anticipated traffic loading conditions. For example, a system in a rural area may only have one omni-directional antenna at each base station, and enough channel modems to support eight simultaneous calls. On the other hand, a base station in a dense urban area may be co-located with other base stations, each having several highly directional antennas, and enough modems to handle forty or more simultaneous calls. It is in these more dense urban areas that cell site capacity is at a premium and must be monitored and managed closely in order to provide the most efficient allocation of limited resources while maintaining acceptable quality of communications.
Sector/cell loading is the ratio of the actual number of users in the sector to the maximum theoretical number that the sector can support. This ratio is proportional to total interference measured at the receiver of the sector/cell. The maximum number of users that the sector/cell can support is a function of the aggregate signal-to-noise ratio (SNR), voice activity, and interference from other cells. The individual subscriber unit SNR depends on subscriber unit speed, radio frequency propagation environment, and the number of users in the system. Interference from other cells depends on the number of users in these cells, radio frequency propagation losses and the way users are distributed. Typical calculations of the capacity assume equal SNR for all users and nominal values of voice activity and interference from other cells. However, in real systems, SNR changes from user to user and frequency reuse efficiency varies from sector to sector. Hence, there is a need to continuously monitor the loading of a sector or cell.
A conventional way to monitor cell site loading conditions is for a person, usually a network engineer or technician employed by a wireless communication service provider, to travel from cell to cell making loading condition readings using specially designed and expensive test equipment. The logged data is then returned to a central processing facility for post-processing and analysis. Some significant drawbacks to this method are that the data can not be evaluated in real-time, and that significant errors are introduced due to propagation effects between the base station and the measurement equipment. Thus, this monitoring method only be used in a time-delayed fashion to take corrective action, such as reassigning resources for the future. It does not enable the service provider to take any real-time action to improve loading conditions and their effect on system performance. Additionally, it requires a person to travel to each site serially, thus providing a discontinuous xe2x80x9chit or missxe2x80x9d estimate of the peak loading conditions and consequent system performance depending on whether the visit coincided with the actual (rather than assumed) peak usage times.
Another possible way of monitoring cell site loading conditions is accessing the performance data logged by the base station, or the base station controller. However, this method requires that scarce base station processing resources be diverted to collect and retrieve the loading data. Additionally, it suffers from the non-real time post-processing problems previously mentioned. It also requires that a person visit each cell site serially to retrieve the data.
One alternate method for monitoring loading in CDMA systems that is known in the prior art is the use of a dedicated channel. However, this solution is very expensive since the capacity of the dedicated channel can not be used for any other purposes. Consequently, a better way to monitor loading of CDMA communication systems is needed. These problems and deficiencies are clearly found in the art and are solved by the invention in the manner described below.
Thus it is desirable to provide a better way to perform load monitoring in CDMA systems wherein a base station determines the total amount of interference in its frequency band that it receives from all the other transmitters in the system. The load monitoring performed this way can be an important aspect of the operation and maintenance of CDMA systems. For example, load monitoring can be used to predict approaching system overloads. It can also be used to control the amount of loading in a system and to establish an admission policy for adding users to the system. The admission policy can apply to new users as well as to users already on the system and being handed off since admission of either kind of user can result in exceeding the system capacity. In addition to limiting the admission of users, steps can be taken to allocate more resources in response to load monitoring. Load monitoring can also be used to determine peak hour activities in CDMA systems.
The present invention is directed to a method and apparatus for monitoring the load on a CDMA communication system having a base station and a plurality of users. A measure of voice activity in the communication system is determined, and a current value of frequency reuse efficiency equal to an initial value of frequency reuse efficiency is provided. A power determination is made according to the determined voice activity and the current value of frequency reuse efficiency. The current value of frequency reuse efficiency is updated using the power determination to provide a new current value of frequency reuse efficiency. The power determination and the update of the frequency reuse efficiency are iteratively repeated until convergence to provide a converged value of frequency reuse efficiency. The load on the communication system is then determined in accordance with the converged frequency reuse efficiency value.
In one embodiment, after the load on the communication system is determined, the admission of new users to the communication system is controlled using the determined load value, e.g., admission of new users is denied when the load is above a threshold. Additionally, load values calculated in accordance with the present invention can be stored in order to collect peak hour activities relating to the communication system.